The reason your lower legs are so skinny is a result of genetics as well as your activity level and muscle mass.
The lower legs have a lot of small muscles in them which by nature will often stay small.
Individual muscle fiber types and overall body composition are also inherited, which influences how easily and how much muscle mass you can gain.
The lower legs have smaller muscles when compared to the upper legs, and these muscles in the lower leg might not naturally be as large as the bigger muscles that are in your thighs.
Also regularly engaging in activities which work your lower legs like jumping, running or even doing calf raises can help to stimulate your muscle growth and increase the size of the muscles in your lower legs.
And proper nutrition and leg workout routines which include calf exercises can also lead to stronger and more developed lower legs.
Muscle mass also naturally declines as you age and so maintaining your muscle strength through exercise is even more important as you get older.
And even being underweight or even having a low percentage of muscle mass can also also lead to thinner legs.
Although genetics do play a huge role in the size of the legs, genetics are not often the only reason for skinny lower legs or skinny legs in general.
Skinny lower legs and skinny legs in general can also result from not eating properly, lifting with improper form and even not training often enough.
If you want to make your lower legs thicker or other parts of your legs thicker you should focus on exercises which target your calf muscles such as doing squats, deadlifts, lunges and calf raises.
Doing exercises regularly can help you build muscle mass and size in your lower legs which can help you get bigger legs and less skinny legs.